Method for operating a fuel cell in the minimal-or partial-load region

ABSTRACT

The invention relates to a method for operating a PEM or DMFC fuel cell in the minimal- or partial-load region. According to the invention, the size of the cell surface, on which the fuel cell reaction takes place, is altered by means of opening or closing of feed channels, which serve to supply reaction medium to the cell surface.

BACKGROUND

The present invention relates to a method for operating a PEM (protonelectrolyte membrane) or DMFC fuel cell (direct methanol fuel cell) inthe minimal or part load range.

In fuel cell systems which have a large number of cells, problems occurwith distributing the reaction media from cell to cell and within a cellin the minimal or part load range. On account of the smaller quantity ofreaction media required in these load ranges, the active cell surface isunder-supplied, resulting in cell failures. Moreover, there are moistureproblems at the active cell surface since, on account of the low flowrate caused by the small quantity of reaction medium, it is impossiblefor sufficient water to be discharged from the fuel cell. To avoid thesecell failures in the lower load ranges, the fuel cell systems areusually operated in higher load ranges and thereby with a greaterquantity of reaction media than is necessary.

Furthermore, in the case of DMFC fuel cell systems, methanolbreakthrough occurs in low load ranges. In the process, the fuelmethanol diffuses through the active cell surface (membrane), with theresult that the efficiency is reduced in this load operation.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for operating afuel cell with which the drawbacks of the prior art can be eliminatedand stabilized operation of the fuel cell associated with a higherefficiency in the lower load range can be achieved.

The present invention provides a method for operating a fuel cell in aminimal or part load range wherein the fuel cell includes a cell surfaceat which a fuel cell reaction occurs and a plurality of passages forcarrying at least one reaction medium and communicating with the cellsurface. The method includes opening or closing at least one of theplurality of passages so as to change a size of the cell surface.

According to the invention, the size of the cell surface at which thefuel cell reaction takes place is changed by opening or closing supplypassages which are used to supply one or both reaction media to the cellsurface. In particular, the supply passages open out into passageregions which are formed into fluid distributor plates (also known asbipolar plates) of the fuel cell. In these passage regions of the fluiddistributor plates, the reaction media supplied are transported to thecell surface.

By means of the method according to the invention, it is possible toopen or close either the passage regions of the reactant or the passageregions of the oxidizing agent. Furthermore, however, it is alsopossible to open or close the passage regions of the two reaction media.

According to the invention, the size of the cell surface can be matchedto the quantity of reactant and/or oxidizing agent which is required fora certain load state of the fuel cell. This prevents under-supply of thereaction media from occurring in certain areas of the cell surface whenthe fuel cell is operated in the lower load range. According to theinvention, the reaction media flow only through the opened supplypassages, which means that the fuel cell is optimally supplied with thesmall quantity of reaction media required for this lower load state ofthe fuel cell. This results in stable fuel cell operation and in anincrease in the efficiency of the fuel cell.

A further advantage is that the methanol breakthrough in DMFC fuel cellsis reduced by means of the inventive matching of the cell surface to therequired quantity of reaction medium.

A further advantage of the present invention consists in the fact thatthe voltage spread at different load states can be reduced.

A load state is understood as meaning an operating state of a fuel cellor a fuel cell stack in which electric power is being taken from thefuel cell or the fuel cell stack. A load therefore corresponds to anelectric power which is being taken off. Electric power (P), electriccurrent (I) and electric voltage (U) are related to one another by meansof the equation P=U·I.

The term voltage spread is understood as meaning the difference betweenthe voltage at full load and the voltage at zero load (fuel cell idlingmode). The cause of the different voltages at different load states isthe current density/voltage characteristic curve which is characteristicof a fuel cell (referred to for short as the i-U characteristic curve,cf. in this respect, by way of example, Larminie/Dicks, “Fuel CellSystems Explained”, Wiley 2001, pp. 37 ff.). The current density (i) isthe current per unit area (A): i=I/A.

At low load, only a low current I, corresponding to a low currentdensity i, flows. According to standard i-U characteristic curves, ahigh voltage is present at low current densities i; in the case wherei=0, this voltage reaches the highest value, the so-called electromotiveforce (EMF). As the load increases and therefore as the current densityincreases, the voltage drops, which is caused by various overvoltages.

In the case of fuel cells or fuel cell stacks of the prior art, it iscustomary for the entire electrochemical active surface of a fuel cellto be operated. In this case, a higher demand for power is usuallysatisfied by increasing the current production of a fuel cell, inaccordance with P=U·I, which in turn is implemented by an increasedsupply of fuel. According to the i-U characteristic curve, the voltagethereby drops.

However, considerable voltage spreads are disadvantageous for the powerelectronics which usually control the removal of power from the fuelcells or fuel cell stacks, since certain electronic components of thepower electronics may be damaged by rapid voltage changes. Furthermore,standard power electronics operate more effectively at approximatelyconstant voltages and thereby contribute to an improved efficiency offuel cells.

According to the invention, an increased demand for power from a fuelcell stack is not satisfied by increased current production on the partof the individual fuel cells, but rather by increasing the activesurface area of the fuel cells. This allows the production of current bythe fuel cells to be increased without the voltage dropping. This allowsa considerable load spread at a low voltage spread.

It is also possible, at a reduced power demand, to reduce the size ofthe electrochemically active surface in the fuel cells. This has theadvantage that the voltage does not rise to an undesirably high level asthe current production drops, as is the case in the prior art. Highvoltages may have two drawbacks. Firstly, certain components of thepower electronics may be damaged. Secondly, at particularly highvoltages (e.g. settings with i<<i_(max), where i_(max) is the maximumcurrent density of a fuel cell), the decomposition voltages of somematerials used in the fuel cells may be reached or even exceeded, whichleads to undesirable electrochemical corrosion to these materials.

According to the present invention, as the power demand drops, the fuelis removed from certain areas of the fuel cell, with the result thatcurrent and voltage drop to zero at these areas, whereas other areascontinue to be operated approximately constantly, producing current andthereby generating a medium voltage. Damagingly high voltages aretherefore avoided.

A further advantage is that the method according to the invention makesit possible to set a lower output voltage of a fuel cell stack for anapproximately constant removal of power, which prevents theabovementioned disadvantageous effects of excessively high voltages onpower electronics and with regard to electrochemical corrosion.

It is also advantageous that the method according to the invention, whenused in fuel cell stacks which are connected in parallel in a fuel cellstack array, allows the output voltages of the individual fuel cellstacks to be matched to the same target value. This allows undesirablecross-currents between the individual fuel cell stacks to be avoided.

It is advantageously also possible for the supply passages leading tothe cooling spaces of the fuel cell which are used to guide a coolingmedium to be opened or closed. This changes the size of the activecooling surface which is in thermal contact with the cell surface.

In an advantageous embodiment of the invention, the supply passagesleading to the cell surface are opened or closed by means of one or moredisplaceable perforated plates. A perforated plate in this case hasapertures which are designed to match the arrangement of the supplypassages. By displacing the perforated plates, it is possible to movethe apertures in a perforated plate into alignment with the supplypassages. In this case, the supply passages are open and it is possibleto supply the corresponding passage regions and therefore the cellsurface with the reaction media. A number of supply passages which ispredetermined according to the apertures can be simultaneously closed oropened by means of the perforated plate. Therefore, the size of the cellsurface is adjusted by opening and closing the supply passages and isthereby matched to the quantity of reaction media in the required loadrange of the fuel cell.

In a further advantageous embodiment of the invention, the supplypassages leading to the cell surface are closed by means of a rotaryblocking slide. Therefore, the supply passages can be opened or closedby rotating the rotary blocking slide.

It is advantageously also possible to close the discharge passages fordischarging one or both reaction media from the cell surface. Thisprevents a reaction medium from flowing back out of a discharge passageto the cell surface which has already been closed by a closed supplypassage. Moreover, it is advantageously also possible to close off thedischarge passages for the cooling medium.

A preferred embodiment of the device according to the invention providesfor plastic tubes with corresponding cutouts to be introduced inside oneor more ports via which fuels or oxidizing agents are fed to a fuel cellstack. The tubes are mounted at the stack ends, for example usingsliding bearings, and at one end are provided with a drive unit. Thisdrive unit has to be able to rotate the tube through 0° to approximately90° or, depending on the design of the port, through 180°, in order toblock off 0 to approximately 90% of the supply passages into a fuelcell. This results in a power drop in a fuel cell from 100% toapproximately 5%.

By means of the method according to the invention, it is possible to setthe size of the cell surface in the range between 5% and 100% of itstotal size. This allows stable operation of the fuel cell at a highefficiency (especially in the case of DMFCs, by reducing the methanolbreakthrough) even in the fuel cell load range of below 10%.

The present invention also relates to a method for operating a PEM orDMFC fuel cell stack in the minimal or part load range, in which methodthe size of the cell surface at which the fuel cell reaction takes placeis changed by opening or closing supply passages which are used tosupply one or both reaction media to the cell surface.

The method allows the fuel cell stack to operate stably and allows theefficiency of the fuel cell stack to be increased, in particular in thecase of DMFCs.

An advantageous variant of the method according to the inventionprovides for the supply of one or both reaction media to be interruptedat zero load, i.e. at i=0, and for the fuel cell stack to beelectrically short-circuited at the same time.

This has the advantage that high voltages, such as for example idlingvoltages, which at zero load may exceed the decomposition voltages ofmaterials used, cannot form, and it is therefore possible to preventelectrochemical corrosion of components of the stack.

The cause of the abovementioned idling voltages at zero load is thatafter the fuel cell stack has been operating under load, reaction mediaare still present in the individual fuel cells and may continue to reactelectrochemically, thereby setting the idling voltage.

According to the method of the invention, therefore, it is preferred forthe fuel cell stack to be short-circuited at zero load after a load hasbeen present, in order to reduce voltages which are present, and for thesupply of one or both reaction media to be interrupted, in order toprevent the electrochemical reaction from continuing.

The short circuit can be executed by means of a suitable device, forexample a switch in a short-circuit line and a discharge resistor.

A further advantageous variant of the method according to the inventionprovides for not only the supply of one or both reaction media to beinterrupted at zero load, i.e. at i=0, and for the fuel cell stack to beelectrically short-circuited at the same time, but also for thedischarge of one or both reaction media to be interrupted.

The simultaneous interruption of supply and discharge passages may berequired in particular when hydrogen is used as fuel, since otherwise asupply from the output side is possible. This is because the reactinghydrogen can generate a vacuum, by means of which further fuel is suckedin from the output side. The same can be true of the cathode side, inparticular if pure oxygen is being used. In the case of operation withreformate gas, for example with an H₂ content of 50 to 60% by weight,and with air, the level of inert components in the gas ensures that thiseffect is greatly reduced.

It is also preferred if the interruption to the supply and/or dischargeof one or both reaction media is effected by closing all the supplyand/or discharge passages.

This has the advantage that switching off at zero load can take placevery quickly and without unnecessary fuel consumption, which furtherincreases the efficiency of the fuel cell stack.

To open or close the discharge passages, it is in principle possible toprovide the same means as those used to open or close the supplypassages.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in more detail below on the basis of drawingsand with reference to examples. In the drawings:

FIG. 1 shows a first exemplary embodiment for carrying out a methodaccording to the invention by means of displaceable perforated plates,in three different operating positions,

FIG. 2 shows a further exemplary embodiment for carrying out a methodaccording to the invention by means of a rotary blocking slide,

FIG. 3 shows an example comparing the efficiency of a DMFC fuel cell inthe lower load range with its full cell surface active compared to areduced active cell surface.

DETAILED DESCRIPTION

FIG. 1 shows a first exemplary embodiment for carrying out a methodaccording to the invention by means of displaceable perforated plates 1.This figure illustrates a sectional view, from the side, through a fuelcell stack 4. The fuel cell stack 4 is formed by stacking a plurality offluid distributor plates 3. Passage regions (not shown), which are usedto distribute the reaction media to the active cell surface (not shown),are formed in the fluid distributor plates 3. An active cell surface isunderstood as meaning in particular a membrane electrode assembly, alsoknown as MEA for short. An MEA comprises an anode, a cathode and aproton-conducting electrolyte membrane arranged between the anode andcathode. Proton transport from the anode to the cathode is ensured bymeans of the proton-conducting electrolyte membrane (PEM). The MEA is inthis case arranged between the fluid distributor plates 3 which aresupplied with the reaction media from the supply passages 2.

The supply passages 2 and therefore the access to the correspondingpassage regions are closed off by means of perforated plates 1. Theperforated plates 1 are arranged perpendicular to the fluid distributorplates 3 and therefore perpendicular to the supply passages 2. Theperforated plates 1 have apertures 5 which are designed to match thearrangement of the supply passages 2. By displacing a perforated plate1, it is possible to align the apertures 5 in the perforated plate 1with the supply passages 2, with the result that it is possible tosupply the corresponding passage region and therefore the active cellsurface. If the apertures 5 in the perforated plate 1 are not alignedwith the supply passages 2, the corresponding passage regions are closedoff and the active cell surface is not supplied with reaction medium.

The left-hand illustration in FIG. 1 shows an arrangement of theperforated plates 1 in which the apertures 5 of the individualperforated plates 1 are aligned congruently with the supply passages 2.

Therefore, the entire passage region in the individual fluid distributorplates 3 of the fuel cell 4 is open and can be supplied with thereaction media.

In the middle illustration in FIG. 1, the left-hand perforated plate 1has been displaced in such a manner that the apertures 5 in theperforated plate 1 are not aligned with the corresponding supplypassages 2 (shown in dashed lines). In the excerpt illustrated,therefore, ⅓ of the supply passages 2 are closed off and the reactionmedia are not flowing through them.

The right-hand illustration in FIG. 1 shows that further supply passages2 are closed off by displacing the second perforated plate 1. Therefore,in this illustration, only ⅓ of the supply passages illustrated areopened, allowing the reaction media to be passed into the passageregions of the individual fluid distributor plates 3 and therefore tothe active cell surface.

FIG. 2 illustrates a further exemplary embodiment for carrying out themethod according to the invention. In this case, a rotary blocking slide6 is illustrated in plan view. The supply passages 2 are connected tothe rotary blocking slide 6, the openings of the supply passages 2 beingsuccessively opened or closed by rotation of the rotary blocking slide6. It is possible to supply the active cell surface through the opensupply passages 2.

From the outer tube 7 of the rotary blocking slide 6 illustrated in FIG.2, a plurality of supply passages 2 branch off to the passage regions ofthe fuel cell and to the active cell surface (not shown). An arc segmentof an inner tube 8, which is formed over a predetermined angular range,is arranged inside the outer tube 7 of the rotary slide 6. The outerradius of the arc segment of the inner tube 8 corresponds to the innerradius of the outer tube 7, the inner tube 8 being mounted rotatablyinside the outer tube 7. By rotating the arc segment of the inner tube8, it is possible to open or close the supply passage 2 leading to thepassage regions of the fuel cell. A supply passage 2 is closed when thearc segment of the inner tube 8 covers this supply passage 2. If the arcsegment of the inner tube 8 is moved away past the opening of the supplypassage 2 by being rotated, the supply passage 2 is open.

Of course, the embodiments of the supply passages explained in FIG. 1and FIG. 2 can also be carried out for the discharge passages.

FIG. 3 illustrates the efficiency of a DMFC fuel cell with respect tothe power of the fuel cell, considering only the load range below 10% ofthe maximum power. FIG. 3 shows how a reduced active cell surface areain the lower power range affects the efficiency of the fuel cell.

In the illustration, A represents the curve of the efficiency of a fuelcell in which the reaction media flow onto the entire cell surface inthe lower power range under consideration. In this case, the efficiencydrops greatly below a power of approximately 8% of the maximum power. Bycontrast, the efficiencies with a reduced active cell surface area areconsiderably higher.

Curve B shows the profile of the efficiency for a fuel cell in which ⅔of the active cell surfaces is being supplied with the reaction media.The efficiency of this fuel cell, below a fuel cell power ofapproximately 8%, is significantly higher than the efficiency of a fuelcell as represented by curve A and only drops significantly below apower of 5%.

If the active cell surface area is reduced to ⅓ of the total surfacearea of the active cell surface (curve C), the efficiency of a fuel cellof this type below a power of approximately 7% is significantly greaterthan the efficiency as represented by curve A, and below a power ofapproximately 5% is even significantly greater than the efficiency asrepresented by curve B. The efficiency (curve C) of this fuel cell onlydrops at a power of 2%.

A significantly higher efficiency can be achieved in the lower loadranges of a fuel cell by means of the method according to the inventionof reducing the active cell surface.

1. A method for operating a fuel cell in a minimal or part load range,the fuel cell including an active cell surface at which a fuel cellreaction occurs and a plurality of supply passages for carrying at leastone reaction medium and communicating with the active cell surface and aplurality of discharge passages discharging the at least one reactionmedium from the active cell surface, the method comprising: opening orclosing the plurality of supply passages so as to change a size of theactive cell surface; and opening or closing the plurality of dischargepassages.
 2. The method as recited in claim 1 wherein the fuel cell is aPEM or DMFC fuel cell.
 3. The method as recited in claim 1 wherein thefuel cell includes an active cooling surface disposed in thermal contactwith the active cell surface and a plurality of coolant supply passagesfor supplying a cooling medium to the active cooling surface and whereinthe method further comprises opening or closing at least one of theplurality of coolant supply passages so as to change a size of theactive cooling surface.
 4. The method as recited in claim 1 wherein theopening or closing is performed using at least one displaceableperforated plate.
 5. The method as recited in claim 1 wherein theopening or closing is performed using at least one rotary blockingslide.
 6. The method as recited in claim 1 wherein the size of theactive cell surface is capable of being adjusted by 5% and 100% of amaximum active cell surface size.
 7. The method as recited in claim 1wherein the passages are supply passages for supplying the at least onereaction medium to the cell surface.
 8. The method as recited in claim 1wherein the passages are discharge passages for discharging the at leastone reaction medium from the cell surface.
 9. The method as recited inclaim 1 wherein the opening or closing includes only closing.
 10. Themethod as recited in claim 1 further comprising interrupting a supply ofthe at least one reaction medium when a load of the fuel cell is zero soas to electrically short-circuit the fuel cell.
 11. The method asrecited in claim 10 wherein interrupting includes interrupting both thesupply and a discharge of the at least one reaction medium.
 12. Themethod as recited in claim 10 wherein the interrupting is performed byclosing all of the plurality of passages.
 13. A device for operating afuel cell in a minimal or part load range comprising: a active cellsurface configured to accommodate a fuel cell reaction; a plurality ofsupply passages for carrying at least one reaction medium andcommunicating with the active cell surface; a plurality of dischargepassages for discharging the at least one reaction medium from theactive cell surface; and an active cell surface adjustment deviceconfigured to open or close the plurality of supply passages and to openor close the plurality of discharge passages.
 14. The device as recitedin claim 13 further comprising: a cooling surface disposed in thermalcontact with the active cell surface; a plurality of coolant passagesfor carrying a cooling medium and in communication with the coolingsurface; and a cooling surface adjustment device configured to open orclose at least one of the plurality of coolant passages.
 15. The deviceas recited in claim 13, wherein the active cell surface adjustmentdevice includes at least one of a perforated plate and a rotary slidedisposed adjacent the plurality of passages.
 16. The device as recitedin claim 14, wherein the cooling surface adjustment device includes atleast one of a perforated plate and a rotary slide disposed adjacent theplurality of coolant passages.